The skin of the human body is an organ that acts as a protective interface with the ambient environment. The epidermis is the outermost layer of the skin. It forms a waterproof, protective wrap over the body's and lips' surfaces and is made up of stratified squamous epithelium which is an underlying basal lamina. The epidermis contains no blood vessels, and cells in the deepest layer obtain nutrients by diffusion from the blood capillaries extending to the upper layers of the dermis. The main type of cells which comprise the epidermis are Merkel cells, keratinocytes with melanocytes and Langerhans cells. The epidermis can be further subdivided into the following strata (beginning with the outermost layer): corneum, lucidum (only in palms of hands and bottoms of feet), granulosum, spinosum, and basale. Cells are formed through mitosis at the basal layer. The daughter cells move up the strata by changing shape and composition as they die due to isolation from their blood source. The cytoplasm is released and protein keratin is inserted. The cells move up to the corneum and slough off through desquamation, which is a process called keratinization taking place over a 27-day period. This keratinized layer of skin and lips is responsible for keeping water in the body and keeping other harmful chemicals and pathogens out while providing a natural barrier to infection. The external layers of the skin change in the number of cellular layers, and while, for example, the facial skin has sixteen cellular layers, the specialized keratinized outer mucosa of the lips only has three to five cellular layers. Always the common factor is that the outer cellular layers are very low in water (circa 20%), while the deeper layers have a much higher water content (circa 80%).
The skin is exposed to several forms of stress including ozone, ultraviolet (UV) radiation, air pollution, pathologic microorganisms, chemical oxidants and topically applied substances. Oxidative stress occurs when some molecules (oxidizing agents) take electrons from the other molecules or atoms. The substances that can exist with missing electrons are called free radicals. Most of these free radicals are oxygen molecules or atoms. Free radicals are highly reactive in that they are ready to give away an electron or to accept one. After the lonesome electron pairs, the atom loses its radical activity, but the atom that has just lost an electron becomes a free radical in turn. These free radical reactions are a necessary part of metabolic processes, however, too many free radicals cause dangerous chain reactions that may destroy cellular composition and may cause damage to DNA, skin proteins and lipids (fats) through these oxidative stresses. The skin of the human body is exposed to oxidative stressors through the environment, bi-products of metabolism, and lifestyle factors such as smoking, alcohol, and UV radiation exposure. These oxidative stresses stimulate the production of unstable molecules otherwise known as reactive oxygen species (ROS) or free radicals. Free radicals are highly reactive molecules that are created as bi-products of normal metabolism (intrinsic) and environmental stressors (extrinsic), and are responsible for cellular damage to the skin. Cellular damage to the layers of the skin can be from free radicals generated from sun light exposure or during metabolic processes in the human body.
Sunlight in a broad sense refers to the total frequency spectrum of electromagnetic radiation given off by the sun. The solar spectrum consists of electromagnetic radiation with a wavelength ranging from 200 to 2500 nm, which includes UV, visible and infrared radiation. The UV portion of the spectrum (200 to 400 nm) is responsible for the most damage to human skin. The UV spectrum may be further broken down by the long wave portion (UVA), and a medium wave portion (UVB). Table 1 below provides a breakdown of the wavelength range and energy per photon for the UVA and UVB portions of the ultraviolet spectrum.
TABLE 1Wavelength rangeEnergy NameAbbreviation in nanometersper photon*Ultraviolet A, long UVA400 nm-315 nm3.10-3.94 eVwave, or black lightUltraviolet B UVB315 nm-280 nm3.94-4.43 eVor medium wave*1 eV-1.6 × 10-19 joule
The UV flux at the Earth's surface has been determined and the erythemal dose to human skin at local noon has been calculated for all latitudes and it varies from 0.0 to 0.4 W/m2. Because of variations in the intensity of UV radiation passing through the atmosphere, the risk of sunburn increases with proximity to the tropic latitudes, located between 23.5° north and south latitude. The noon erythemal dose to human skin at 23.5° latitude is 0.25 W/m2. FIG. 1 shows the depth of penetration for UVA and UVB radiation through the skin. From FIG. 1, it can be seen that the sun burning radiation (wavelengths shorter than 315 nanometers) is nearly all absorbed in the epidermis, which is where UVB radiation has it most harmful effects. Wavelengths longer than 315 nanometers (UVA) is nearly all absorbed in the epidermis and dermis, which is where UVA radiation has it most harmful effects.
Upon absorption of UV radiation energy by photo reactive molecules (chromophore), a photo chemical reaction is induced. The absorption of the radiation energy by the chromophores (P) in its ground state will occur. The formation of the excited (usually triplet) state (3P) molecule occurs. The excited state molecules then participate in oxygen dependent processes (i.e., photodynamic processes) in two major pathways: Type I or Type II reactions, both of which result in cytotoxic injury to the skin. The cytotoxic injury by the UV radiation causes mutations and death in cells of the skin. The Type I reaction involves transfer of an electron or a hydrogen atom to the excited state photosensitizer (3P) resulting in the formation of free radicals (Equation 1 below), which leads to oxidative reduction reactions that results in hydrogen peroxide formation and subsequent cell damage (Equations 2 and 3 below).3P+RH→PH.+R.  (1)PH.+PH.→P+PH2  (2)PH2+O2→P+H2O2  (3)3P+O2→P+1O2  (4)
The interaction of 3P with ground state oxygen could result in the formation of superoxide anions (O2.)—, which in turn, can be converted into highly reactive and cytoxic hydroxyl radicals (OH.). The Type II reaction depicted above is also known as an energy transfer process. Transfer of energy to ground state oxygen results in the formation of singlet oxygen (1O2), which is highly reactive and has a lifetime of 50 nanoseconds (See equation 4 above). Cytotoxic injury occurs upon singlet oxygen-induced oxidation of amino acids and unsaturated fatty acids with interaction of the latter resulting in the formation of hydroperoxides, which initiate lipid and protein oxidation.
Exposure to the UV spectrum of sunlight through the mechanisms described above has been associated with skin cancer. Excessive UV radiation from sun exposure causes DNA damage, inflammation, erythema, sunburn, immunosuppression, photoaging, gene mutations, and skin cancer. Sunburn can also be caused by pharmaceutical products that sensitize some users to UV radiation. Certain antibiotics, oral contraceptives, and tranquilizers have this effect. In general, people with fair hair and/or also freckles have a greater risk of sunburn than others because of their lighter skin tone and low melanin in the skin. Upon DNA damage, tumor suppressor protein undergoes phosphorylation and translocation to the nucleus and aids in DNA repair or causes apoptosis. Excessive UV exposure overwhelms DNA repair mechanisms. UV radiation is a common cause of melanoma and sunburn. Sunburn is caused by direct DNA-damage, whereas melanoma is caused by indirect DNA-damage. Protecting against sunburn does not necessarily protect against melanoma, however protection of the skin from sunlight radiation is highly desirable. The preferred skin protection method against UV radiation damage is clothing, including hats. Moderate sun tanning without burning can also prevent subsequent sunburn, as it increases the amount of melanin, a skin photoprotectant pigment that is the skin's natural defense against overexposure. The erythemal diurnal dose rate for sun tanning is dependent on latitude and time of the solar day.
Sunscreens can help prevent sunburn, although they may not effectively protect against malignant melanoma, which either is caused by a different part of the UV spectrum or is not caused by sun exposure at all. Sunscreens utilize a combination of antioxidants, which are generally selected haphazardly without taking into consideration the physicochemical factors which will help prevent cytotoxic damage. The use of sunscreen is known to prevent the direct DNA damage that causes sunburn and the two most common forms of skin cancer, basal-cell carcinoma and squamous cell carcinoma. However, if sunscreen penetrates into the skin, it promotes indirect DNA damage, which causes the most lethal form of skin cancer, malignant melanoma. The existing SPF label value for sunscreen is based according to the formula SPF/Minimal Erythemal Dose 2/3 where the effective erythemal dose is 200 W/m2. At a dose rate 0.250 W/m2 it takes 800 seconds of local noon exposure at a latitude of 23° N (Key West, Fla.) to be exposed to 1 MED of UV radiation. In addition to UV mediated photosensitization, the sunscreen filter is absorbed into the skin, and prevents sunburn, but also increases the amount of free radicals, which in turn increases the risk for malignant melanoma. The harmful effect of photo-excited sunscreen filters on living tissue has been shown in many photo-biological studies. Whether sunscreen prevents or promotes the development of melanoma depends on the relative importance of the protective effect from the topical sunscreen versus the harmful effects of the absorbed sunscreen. Therefore, it is highly desirable to develop compositions of antioxidants that will prevent UV induced oxidative damage to the outer layers of the skin.
In order to determine the various physiological effects of UV radiation on human skin, it is necessary to develop an understanding of the depth of penetration of the different UV wavelengths into the skin. But this is difficult because the skin is made up of optically inhomogeneous layers having different properties and varying in thickness and structure from one part of the body to another. An understanding of the different reaction mechanisms taking place with the skin from UV radiation and how the structure and chemical composition of skin varies from these UV induced oxidative stresses is important to developing skin treatment compositions that protect the skin from harmful UV radiation. Therefore, there is also a need to develop methods that aid in the screening and selection of antioxidant combinations that may be used to protect the skin from the harmful effects of UV radiation and other environmental and metabolic processes.